SYNCHRONOUS CONVERTERS

273

influence on the heating and the output of the machine, a single
autotransformer is preferable because of its simplicity. In
three-wire distribution systems the latter is practically always the
case, that is, the load fairly balanced and the neutral current
small.

The size of the autotransformers depends upon the amount of
unbalanced power, that is, the maximum difference between the
load on the two sides of the three-wire system, and thus equals
the product of neutral current iQ and voltage e between neutral
and outside conductor; that is, in the three-wire system of vol-
tage e per circuit, voltage 2 e between the outside conductors,
and maximum current i in the outside conductors, the generator
power rating is

p = 2 ei.

Let now ^o = maximum unbalanced current in the neutral—
usually not exceeding 10 to 20 per cent, of i—and using a single
autotransformer, connected diametrically across the armature,
Fig. 146, the maximum of the alternating voltage which1 it re-
ceives is 2 e, and its effective voltage therefore e V2. As the
neutral current iQ divides when entering the autotransformer, the

current in the compensating winding is ~~ (neglecting the small

JU

exciting current), and the volt-ampere capacity of the autotrans-
former thus is

and

Po =

2° =
p

= 0.354 v •

i

Even with the neutral current equal to the current in the out-
side conductor, or the one side of the system fully loaded, the
other not loaded, the autotransformer thus would have only
35.4 per cent, of the volt-ampere capacity of the generator, and
as an autotransformer of ratio 1 -r- 1 is half the size of a trans-
former of the same volt-ampere capacity, in, this case the auto-
transformer has, approximately, the size of a transformer of
17.7 per cent, of the size of the generator.

With the maximum unbalancing of 20 per cent., or •-.-- = 0,2,

i